A lighting device for an instrument for a vehicle is for example known in Japanese Unexamined Patent Publication No. 2004-291949. The lighting device is an edge-type lighting device in which a light emitting diode is optically coupled at an edge of the light conducting plate. In the edge-type lighting device, the light emitting diode emits light toward a light incident surface of the light conducting plate. The light conducting plate diffuses the light therein and radiates the diffused light uniformly from its light radiating surface toward a dial board of the instrument. Thus, the dial board is visible with uniform brightness.
In such an edge-type lighting device, an on-chip light emitting diode is mounted to and electrically connected to a sub-circuit board, which is provided only for the light emitting diode, i.e., provided separately from a main circuit board for controlling respective components of an instrument. A light conducting plate is arranged parallel to the main circuit board. The sub-circuit board is for example held perpendicular to the light conducting plate such that an optical axis of the light is substantially parallel to the light conducting plate. Here, the sub-circuit board functions to supply electric power to the light emitting diode and to radiate heat generated from the light emitting diode.
Japanese Unexamined Patent Publication No. 2001-160312 also discloses a lighting device. In the lighting device, an on-chip type light emitting diode module is mounted to a circuit board. In the lighting device, the circuit board is made of metal such as aluminum for improving radiation of heat generated from the light emitting diode module. Electrodes (connection terminals) of the light emitting diode module and lands (conductive patterns) formed on the circuit board function as power supplying means and heat radiating means. Namely, the heat generated from the light emitting diode module is transferred to the circuit board through the electrodes and the lands and radiated from the circuit board.